Effects of several sea anemone and scorpion toxins on excitability and ionic currents in the giant axon of the cockroach.

The membrane effects of 4 sea anemone and 6 scorpion toxins have been studied under current clamp and voltage clamp conditions. Micromolar concentrations of the purified toxins were applied externally on single giant axons of the american cockroach. Periplaneta americana in a double oil-gap arrangement and the effects on the resting potential, action potential and underlying currents analysed. The 4 sea anemone toxins (Condylactis toxin, Anemonia toxin 2, Anthopleurin toxin A and Parasicyonis toxin) were found to considerably prolong the action potential. This effect is frequency dependent and long plateau spikes (100-500 ms in duration) are consistently seen for frequencies lower than 0.2 Hz. This effect is due to a considerable delay in the turning-off of the sodium current during square membrane depolarizations associated, for large concentrations, with a decrease in the potassium conductance. Toxin effects on the sodium current are not prevented by pretreatment with STX. From the 4 purified toxins extracted from the venom of the scorpion, Androctonus australis Hector, 3 (Mammal toxins 1 and 2 and crustacean toxin) were found to have sea anemone toxin like effects and to induce long duration plateau action potentials. As for sea anemone toxins, this effect is due to a lengthening of the falling phase of the sodium current associated with a small decrease in the potassium conductance. The 4th toxin (insect toxin or ITAaH) depolarizes the membrane and induces repetitive firing of short action potentials.(ABSTRACT TRUNCATED AT 250 WORDS)